The Long Term Evolution (LTE) telecommunication standard uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete fourier transform (DFT)-spread OFDM in the uplink. Downlink refers to transmissions from a radio base station to a user equipment unit served by the base station, while uplink refers to transmission from the user equipment unit to the base station. In an OFDM system, data is sent simultaneously over a group of orthogonal subcarrier frequencies. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms in duration. Each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Carrier Aggregation
The recently adopted LTE Rel-8 standard supports bandwidths up to 20 MHz. However, in order to meet the International Mobile Telecommunications (IMT)-Advanced requirements, the 3rd Generation Partnership Project (3GPP) has initiated work on LTE Rel-10. One goal of LTE Rel-10 is to support bandwidths larger than 20 MHz, although it would be desirable for LTE Rel-10 to be backwards compatibile with LTE Rel-8, including spectrum compatibility. Thus, an LTE Rel-10 carrier that is wider than 20 MHz should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC).
In particular, for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is also desirable to ensure an efficient use of a wide carrier by legacy terminals. That is, it should be possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 frequency space. One straightforward way to obtain this would be by means of Carrier Aggregation. Carrier Aggregation implies that an LTE Rel-10 terminal can receive multiple component carriers, where each of the component carriers may have the same structure as a Rel-8 carrier. In a Rel-8 structure, all Rel-8 signals, e.g. primary and secondary synchronization signals, reference signals, and system information are transmitted on each carrier.
Carrier Aggregation is illustrated in FIG. 3. As shown therein, five component carriers CC1 to CC5 each having a bandwidth of 20 MHz may be aggregated to provide a channel have an aggregated bandwidth of 100 MHz. Although illustrated in FIG. 3 as being contiguous in frequency, it will be understood that component carriers that are not contiguous in frequency can be aggregated to provide an increased bandwidth channel.
The number of aggregated component carriers, as well as the bandwidth of the individual component carrier, may be different for the uplink and downlink. In a symmetric configuration, the number of component carriers in the downlink and the uplink is the same. In an asymmetric configuration, the number of component carriers in the uplink is different from the number of component carriers in the downlink. It is important to note that the number of component carriers configured in a cell coverage area may be different from the number of component carriers seen by a terminal. A terminal may, for example, support more downlink component carriers than uplink component carriers, even though the network offers the same number of uplink and downlink component carriers.
During initial access, an LTE Rel-10 terminal may behave in a manner that is similar to an LTE Rel-8 terminal. Upon successful connection to the network, a user equipment unit may, depending on its capabilities and the capabilities of the network, be configured to use additional component carriers in the uplink and downlink. The configuration is based on radio resource control (RRC) signaling. Due to the heavy signaling and the relatively slow speed of RRC signaling, it is expected that user equipment units may be configured to handle multiple component carriers, even though not all of them may be used at any given time. If a user equipment unit is configured to use multiple component carriers, it would have to to monitor all downlink component carriers for control channels, such as the Physical Downlink Control Channel (PDCCH) and the Physical Downlink Shared Channel (PDSCH). This would require the user equipment unit to support a wider receiver bandwidth, higher sampling rates, etc., which may result in high power consumption.
To mitigate these issues, LTE Rel-10 supports both configuration and activation of component carriers. The user equipment unit may monitor only configured and activated component carriers for the PDCCH and PDSCH. Since activation is based on Medium Access Control (MAC) control elements, which are faster than RRC signaling, activation/de-activation can be based on a number of component carriers that are currently required to fulfill the data rate needs at a given time. Upon arrival of large amounts of data, multiple component carriers may be activated and used for data transmission, and then de-activated when no longer needed. In most cases, all but one component carrier, namely the downlink Primary component carrier (DL PCC), can be de-activated. Activation therefore provides the possibility to configure multiple component carriers but only activate them on an as-needed basis. Most of the time, a terminal would have one or very few component carriers activated, potentially resulting in a lower reception bandwidth and thus lower battery consumption.
Scheduling of a component carrier is done on the PDCCH via downlink assignments. Control information on the PDCCH is formatted as a Downlink Control Information (DCI) message. In LTE Rel-8, a user equipment unit only operates with one downlink component carrier and one uplink component carrier. The association between downlink assignment, uplink grants and the corresponding downlink and uplink component carriers is therefore straightforward. In LTE Rel-10, two modes of Carrier Aggregation should be distinguished. The first case is very similar to the operation of multiple Rel-8 terminals. A downlink assignment or uplink grant contained in a DCI message transmitted on a component carrier is either valid for the downlink component carrier itself or for an associated (either via cell-specific or terminal specific linking) uplink component carrier. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI containing a downlink assignment with CIF is valid for that downlink component carrier indicted with CIF and a DCI containing an uplink grant with CIF is valid for the indicated uplink component carrier.